The invention relates to a process for producing a nitric acid with the concentration of 75 to 99.9% from a diluted, respectively an impure nitric acid, wherein a nitric acid of a concentration from about 45 to 70% is brought into contact with a fluid extraction medium to prevent formation of a nitric acid-water-azeotropic mixture, which is rectified above normal pressure and/or normal pressure and/or vacuum and wherein the vapors of the concentrated nitric acid are condensed to thereby obtain a concentrated nitric acid, and wherein additionally, the extraction medium is regenerated and by means of reconcentration returned to the extractive rectification; as well as an installation for carrying out a preferred embodiment of such a process.
Nitric acid of higher concentration, oftentimes more than 98% by weight HNO3, is required for many organic reactions such as, for example nitrogenizations.
Conventional production methods produce nitric acid of about 65 to 67% by weight HNO3. Thus, an additional process is required in order to obtain highly concentrated nitric acid. The most suitable process for concentrating nitric acid beyond the azeotropic point is the extractive rectification. Thereby, liquid extraction media are used, in particular concentrated sulfuric acid or aqueous solutions of magnesium nitrate solutions, wherein the extraction medium used is predominantly sulfuric acid.
The thermodynamic bases of the extractive rectification utilizing an extraction medium are as follows:
The component system, nitric acid/water exhibits at ambient pressure (1 bar absolute) a maximal azeotropic mixture with a boiling point temperature of 121.8° C. at 69.2% weight HNO3. For nitric acid concentrations below the azeotropic point, water is the more volatile component. Above the azeotropic point, HNO3 is contained in the vapor phase in higher concentration. For production of highly concentrated nitric acid, it is necessary to circumvent the azeotropic point or respectively to overcome it. It has been known for a long time that with the aid of extraction medium, such as for example sulfuric acid or aqueous magnesium nitrate solutions, the relative volatility of water can be reduced. For higher extraction medium concentrations, such as for example sulfuric acid concentrations of higher than 50% by weight H2SO4, the formation of an azeotropic point is suppressed. Thus, concentrated nitric acid can be distilled therefrom.
The basic process of concentrating nitric acid is by Pauling (DE 305553, DE 1056095). The process is industrially exploited on a large scale, in technically varying installations. A modern industrial method which is carried out with the extraction medium sulfuric acid is described in the following paragraph for a better understanding of the invention (Schott Engineering GmbH, printed to 60014 D. 8895.0; Comp. FIG. 1). The actual process is essentially carried out in a concentration column (RK in FIG. 1), into which the concentrated sulfuric acid is supplied and somewhat below, the diluted nitric acid. By mixing the components, mixing-and condensation temperatures occur in the column. These amounts of heat are however not sufficient to cover the required energy requirement for the process. Thus, at the foot of the column, steam (“stripping steam”) is directly blown into. Aside from its function as a heat carrier, this direct steam effects that the nitric acid is substantially stripped from the diluted sulfuric acid that is running off. The vapors of the concentrated nitric acid, which are given off at the head of the column (“HNO3 exhaust vapors”) also contain nitrogen oxides, which are formed by the thermal decomposition of nitric acid. These nitrogen oxides (essentially NO2) are at the same time condensed within the HNO3 exhaust vapors to red smoking nitric acid. In order to obtain the condensed highly concentrated nitric acid in colorless form, the nitric gases contained in the acid are stripped off by air in a countercurrent downstream column, a so-called bleaching column. The waste gases, which contain nitrogen oxides, are cleaned in a subsequent NO2 absorption step.
A complete installation for producing highly concentrated nitric acid comprises aside from the installation for the nitric acid rectification, additionally an installation branch for the extraction medium reconcentration. The installation parts for the afore-described bleaching of the smoking nitric acid, as well as the downstream NOx absorption are also added. It is also appropriate to provide a pre-concentration for the nitric acid to be concentrated (“feeding acid”) having a concentration of less than about 58% by weight HNO3. Beyond that, it could also be appropriate to purify the nitric acid through steaming. All parts of the entire installation have to be optimally designed in order to fulfill the required specification with respect to product quantity and wastewater purity with minimal use of resources. The resource consumption and specifically the heating energy requirement decidedly determine the operational cost of the process.
In order to minimize the resources consumption, the required heating energy has to be supplied and utilized optimally in the process. In the classic process, the supply of heating energy is effected directly with steam, the stripping steam. Early on, it was tried to save on direct steam since it could be shown that thereby the requirement of extraction medium (sulfuric acid) can be minimized.
In practice, several solutions have been carried out. In the process where aqueous magnesium nitrate solutions is used as extraction medium, early on, a sump heater was utilized (cf. Ullman's Enzyclopedia of Technical Chemistry; 4th Edition, Volume 20, p. 325). In the process wherein sulfuric acid is used, a so-called intermediate heater, also called a column heater and has being utilized for a long time (Ullman's Encyclopedia of Technical Chemistry; 4th Edition, Volume 20 p. 325).
Furthermore, it is known that it is possible to save heating energy when reflux (internal and/or external) is reduced in the extraction rectification column. An internal reflux occurs for example, when the entry temperature of the extraction medium is lower than the boiling point temperature of the three component mixture, so that a certain amount of heat is first taken up by the extraction medium by partial condensation from the components of the vapor phase.
In the conventional methods, where pure extraction medium is deposited at the head of the extractive rectification column, such as for example sulfuric acid, this extraction medium will first condense nitric acid so that the composition of the fluid mixture is moved into the direction of the equilibrium with the vapor phase. It follows, that the extraction medium has to be deposited in sufficiently cold condition to thereby effect an elevated internal reflux. This leads to an additional amount of heat which in turn can be supplied by means of additional heating energy to the extractive rectification.
It is known that by supplying liquid nitric acid (for example, a partial amount of the nitric acid to be concentrated) to the extraction medium, the condensation behavior of the nitric acid in the column for the extractive rectification can be influenced. Accordingly, EP 0330351 81 (“Günkel”) described an admixture of the nitric acid to be concentrated to a partial stream of the extraction medium. A disadvantage of the method described by Günkel is how ever, that the mixture stream of nitric acid to be concentrated and extraction medium is first input below a further amount pure extraction medium already input at an upper location. Thus, the afore-described desired effect of reducing the internal reflux is not attained in the process described in EP 0 330 351.
A further disadvantage of the process according to Günkel is that the entire process comprising also the reconcentration of the extraction medium, the required energy has already been completely supplied to the extractive rectification, although the energy requirement of the extractive rectification is substantially lower than the energy requirement of the extraction medium-reconcentration. With an energy supply of this type, a high amount of extraction medium is necessary for circulation and a large minimum diameter of the extractive rectification column is required.
In the process as described by Günkel 51 to 99% of the amount of nitric acid to be concentrated is fed into the extractive rectification in totally vaporized form. Through this type of input, the amount of steam in the column is substantially raised, so that in connection with the raised amount of extraction medium in circulation, columns of greater diameter are required for the extractive rectification.
A further disadvantage of the process described by Günkel is that at the head of the column a substantially higher reflux amount of highly concentrated nitric acid is required in order to obtain highly concentrated nitric acid vapor containing 99 to 99.9% by weight HNO3.
Until now, in all installations that have been established for highly concentrating subazeotropic nitric acid (weak acid) to nitric acid having more than 90% by weight, preferably more than 98% by weight (“hico-acid”) the energy requirement, such as for example, heating steam and cooling water determined the production costs substantially. Thus, in larger installations as they were commonly established in the immediate past, (with sump heater and/or intermediate heater) starting from an HNO3 of 67% by weight, heating steam amounts of about 1.5 kg per kg produced 99% by weight of nitric acid were required.